1. Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to a top emission active matrix organic electroluminescent device and a fabricating method thereof.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are commonly used for flat panel displays (FPDs) because they are lightweight and consume relatively low amounts of power. However, LCD devices are not light-emitting displays. As such, LCDs have several disadvantages including dim displays, poor contrast ratios, narrow viewing angles and small display sizes. Accordingly, new FPDs, such as organic electroluminescent (EL) devices, have been developed to solve these problems. Organic EL devices are light-emitting displays that possess a wider viewing angle and a better contrast ratio than LCD devices. Furthermore, since no backlight is required for an organic EL device, organic EL devices generally are both lighter and thinner than LCD devices, and consume less power. Organic EL devices may be driven with a low direct current (DC) voltage that permits a faster response speed than LCD devices. Moreover, since organic EL devices are solid-phase devices, unlike LCD devices, they can better withstand external impacts and possess a greater operational temperature range. In addition, organic EL devices may be manufactured more cheaply than LCD devices or plasma display devices (PDPs) because organic EL devices require only deposition and encapsulation apparatus. Organic EL devices may be either top emission organic EL devices or bottom emission organic EL devices depending on the direction of the emitted light.
A passive matrix design that does not use thin film transistors (TFTs) may be used for organic EL devices. In passive matrix organic EL devices, scanning lines and signal lines perpendicularly cross each other in the form of a matrix. A scanning voltage is sequentially applied to the scanning lines to operate each pixel. The voltage applied to a pixel when its scan line is selected should be the voltage required to produce the desired average brightness for the pixel multiplied by the number of the scanning lines. Accordingly, as the number of scanning lines increases, the applied voltage and current demanded by the passive matrix organic EL device increase. Therefore, a passive matrix organic EL device is inadequate for a large high-resolution display because the device has high power consumption, which may cause the device to fail more rapidly.
Since passive matrix organic EL devices are disadvantaged in display resolution, power consumption and expected life span, active matrix organic EL devices have been developed as next-generation display devices that provide high resolution over a large display area. In active matrix organic EL devices, a TFT, disposed at each sub-pixel, is used as a switching element to turn the sub-pixel ON or OFF. Specifically, a first electrode, which is connected to the TFT, is turned ON or OFF by the sub-pixel, and a second electrode, which faces the first electrode, functions as a common electrode. The sub-pixel may retain a voltage applied to the sub-pixel by storing charge in a storage capacitor. The storage capacitor may drive the device until a new scan cycle occurs, and may allow the voltage applied to a sub-pixel to remain the same regardless of the number of scanning lines. Since an equivalent brightness is obtained with lower current demands, active matrix organic EL devices allow larger displays consuming less power and providing higher resolution to be made.
FIG. 1 is an equivalent circuit diagram showing a basic pixel structure of an active matrix organic electroluminescent device according to the related art. In FIG. 1, a scanning line 1 is arranged along a first direction, and a signal line 2 and a power line 3 spaced apart from each other are arranged along a second direction perpendicular to the first direction, thereby defining a pixel region P. A switching TFT TS, which is an addressing element, is connected to the scanning line 1 and the signal line 2. A storage capacitor CST is connected to the switching TFT TS and the power line 3. A driving TFT TD, which is a current source element, is connected to the storage capacitor CST and the power line 3. An organic EL diode DEL is connected to the driving TFT TD. When a forward current is applied to the organic EL diode DEL, an electron and a hole are recombined to generate an electron-hole pair through the P(positive)-N(negative) junction between the anode providing the hole and the cathode providing the electron. The electron-hole pair has a lower energy than the separated electron and hole. Thus, the recombination of the electron and the hole causes light to be emitted as a result of the energy difference. The switching TFT TS adjusts the forward current through the driving TFT TD and stores charges in the storage capacitor CST.
FIG. 2 is a cross-sectional view of a bottom emission organic electroluminescent device according to the related art. FIG. 2 shows one pixel region including red, green, and blue sub-pixel regions. In FIG. 2, a first substrate 10 faces and is separated from a second substrate 30. A peripheral portion of the first and second substrates 10 and 30 is sealed with a seal pattern 40. A TFT T is formed at each sub-pixel region Psub on an inner surface of the first substrate 10. A first electrode 12 is connected to the TFT T in each sub-pixel region. An organic electroluminescent layer 14 including luminescent materials that are red, green, or blue is formed on the TFT T and the first electrode 12. A second electrode 16 is formed on the organic electroluminescent layer 14. The first and second electrodes 12 and 16 apply an electric field to the organic electroluminescent layer 14. An adhesive (not shown) and a moisture absorbent material (not shown) are formed on an inner surface of the second substrate 30 to shield the device from external moisture. In a bottom emission organic electroluminescent device, a first electrode 12 functioning as an anode is made of a transparent conductive material, and a second electrode 16 functioning as a cathode includes a metallic material with a low work function. Here, the organic electroluminescent layer 14 is composed of a hole injection layer 14a, a hole transporting layer 14b, an emission layer 14c, and a electron transporting layer 14d which cover the first electrode 12. In the emission layer 14c, red, green, and blue emissive materials are alternately disposed at adjacent sub-pixel regions. For example, in FIG. 3, green emissive material is disposed at sub-pixel Psub, while the adjacent sub-pixels have red emissive material and blue emissive material, respectively.
FIG. 3 is a cross-sectional view showing one sub-pixel region of a bottom emission organic electroluminescent device according to the related art. In FIG. 3, a TFT T having a semiconductor layer 62, a gate electrode 68, a source electrode 80 and a drain electrode 82 is formed on a substrate 10. The source electrode 80 of TFT T is connected to a storage capacitor CST. The drain electrode 82 of TFT T is connected to an organic electroluminescent (EL) diode DEL. The storage capacitor CST includes a power electrode 72 facing a capacitor electrode 64. An insulating layer is interposed between the power electrode 72 and the capacitor electrode 64. The capacitor electrode 64 includes the same material as the semiconductor layer 62. The TFT T and the storage capacitor CST are referred to as an array element A. The organic EL diode DEL includes a first electrode 12 facing a second electrode 16, and an organic EL layer 14 interposed between the first electrode 12 and the second electrode 16. The source electrode 80 of the TFT T is connected to the power electrode 72 of the storage capacitor CST, and the drain electrode 82 of the TFT T is connected to the first electrode 12 of the organic EL diode DEL. The array element A and the EL diode DEL are formed on the same substrate in the organic electroluminescent device according to the related art.
FIG. 4 is a flow chart showing a fabricating process of an organic electroluminescent device according to the related art. In a first step, array element is formed on a first substrate. The array element includes a scanning line, a signal line, a power line, a switching TFT, and a driving TFT. The signal line is spaced apart from the power line, and the signal line and the power line each cross the scanning line. The switching TFT is disposed at the crossing point of the scanning line and the signal line. The driving TFT is disposed at the crossing point of the scanning line and the power line.
In a second step, a first electrode of an organic EL diode is formed over the array element. The first electrode is connected to the driving TFT of its respective sub-pixel region.
In a third step, an emission layer of the organic EL diode is formed on the first electrode. If the first electrode is designed to function as an anode, the organic EL layer may be composed of a hole injection layer, a hole transporting layer, an emission layer, and an electron transporting layer.
In a fourth step, a second electrode of the EL diode is formed on the organic EL layer. The second electrode is formed over an entire surface of the first substrate to function as a common electrode.
In a last step, the first substrate is encapsulated with a second substrate. The second substrate protects the first substrate from external impacts and prevents damage of the organic EL layer caused by air. A moisture absorbent material may be included in an inner surface of the second substrate.
The organic EL device according to the related art is fabricated by encapsulating the first substrate including the array element and the organic EL diode with the second substrate. Since the production yield of the organic EL device is equal to the production yield of the array element multiplied by the production yield of the organic EL diode, the production yield for an organic EL device is limited by the process for the organic EL diode. Even if the array element is satisfactorily fabricated, the organic EL device may be faulty because the organic EL layer is defective. Accordingly, the expense of fabricating an array element properly and the associated material cost are lost and the production yield is reduced when organic EL diodes are improperly fabricated in an organic EL device according to the related art.
Bottom emission organic EL devices have the advantages of high encapsulation stability and high process flexibility. However, bottom emission organic EL devices are ineffective for high resolution devices because they have poor aperture ratios. In contrast, a top emission organic EL device has a higher expected life span since it is easy to fabricate and has a high aperture ratio. However, in a top emission organic EL device, the cathode is generally formed on the organic EL layer. As a result, the transmittance and optical efficiency of a top emission organic EL device are reduced because of a limited number of materials that may be selected. When a thin film protection layer is used to minimize the transmittance reduction, the top emission organic EL device is not sufficiently shielded from ambient air.